1. Field of the Invention
The present invention relates to the preparation of diesters of hexene-1,6-dioic acid. These diesters can be hydrogenated to the corresponding diesters of adipic acid or adipates, which can in turn be hydrolyzed to produce adipic acid. (Adipic acid is one of the basic raw materials for nylon 66; for this reason alone, any novel process for the ultimate synthesis of such diacid and/or derivative thereof is of fundamental worth).
The present invention more especially relates to the preparation of diesters of 3-hexene-1,6-dioic acid by reacting carbon monoxide and an alcohol with at least one butene compound containing two acyloxy substituents in the presence of a catalyst based on palladium.
2. Description of the Prior Art
The preparation of a monoester of an alcohol and of a butenoic acid by reacting carbon monoxide and an alcohol with a monoester of an allyl alcohol, in the presence of a palladium-based catalyst, is described in U.S. Pat. No. 3,367,961. Ethyl vinylacetate can thus be prepared from allyl acetate, carbon monoxide and ethanol in the presence of palladium chloride and of palladium deposited on charcoal.
In U.S. Pat. No. 4,611,082 it is also indicated that the carbonylation of a solution of 1,4-diacetoxy-2-butene in a polar and nonbasic aprotic solvent selected from among the nitriles, bis(2-methoxy)-2-butene, bis(2-methoxyethyl) ether and methylene chloride, at 80.degree. to 140.degree. C. in the presence of a transition metal halide, is not observed in practice and that, in the presence of an alcohol, the rates increase and are comparable with those observed in the case of the carbonylation of 2-butene-1,4-diol. Concerning said latter substrate, it is also indicated that it does not enable satisfactory yields of linear carbonylation products to be expected under the aforementioned conditions and, in this context, substrates substituted by alkoxy groups in the 1,4-position are the preferred.
Moreover, 1,4-diacetoxy-2-butene is easily prepared by the acetoxylation of butadiene. It would, therefore, be highly desirable to provide this art with a process making it possible to produce, with a high efficiency, the diesters of 3-hexene-1,6-dioic acid from 1,4-diacetoxy-2-butene, for example, and, more generally, from butenes disubstituted by acyloxy groups.